The present invention relates to the field of electrostatic painting as practiced on a looped chain-type conveyor, and more particularly, to a method and apparatus for cleaning paint deposits from the hooks used to connect process parts to the conveyor.
Electrostatic painting is a method for improving the efficiency of depositing paint on a process part by placing an electrical charge on the paint and grounding the part, so that there is a mutual attraction. The process first came into commercial use after World War II in the automobile industry and has since become a commonly accepted practice. Generally, products to be painted are attached to the chain of a looped overhead or in-floor conveyor, and pass through an enclosure for spray painting. In a variation of this process, the product is dipped in paint. Usually, the conveyor is electrically connected to ground, and the paint spray gun or tank is connected to the charged side of a grounded D.C. system. Product supporting hooks, which may be simple, hook shaped members, or elaborate wire frames adapted to support a specific part or product, are attached to the conveyor chain at spaced intervals. In this manner, the hook members become connecting links, joining the product into the electrostatic system. As a result, the paint hooks attract their share of paint with each pass through the system. These layers of paint will build-up to a point that reliable electrical connections cannot be made between the hooks and the process parts, disabling the effect of the electrostatic field.
The paint build-up can be removed in various ways, the most efficient being by use of a burn-off oven, preferably a controlled atmosphere burn-off oven as described in my U.S. Pat. No. 5,189,963. In this process, the hooks are removed from the conveyor, stacked on carts designed for the purpose and oven baked to burn-off the paint build-up. The burn-off oven method for hook cleaning is preferred as being environmentally acceptable and economical, but if a burn-off oven is not available, the paint hooks can be cleaned by media blasting, chemical stripping or manual methods.
In the burn-off process, oven heat is controlled, so as to reduce the paint to ash in an environmentally acceptable manner. Titanium dioxide, included in virtually all paint formulations, is unaffected by the heat of the oven, so that a residual titanium dioxide coating remains on the surface of the hooks. This coating is non-conductive and must be removed from the hooks, at least in the areas of product contact, by an additional step involving a mechanical, chemical or manual process. Even with the additional step, use of the burn-off oven for hook cleaning is more economical than chemical stripping or other alternatives.
A first object of the present inventions is therefore, to provide effective methods and apparatus for removal of paint from electrostatic painting conveyor hooks. A second object is that the methods and apparatus provided also be effective for removal of the residual, non-conductive titanium dioxide. A third object of the present inventions is to provide methods and apparatus more efficient and economical than current practices. Yet a fourth object is to provide environmentally acceptable methods and apparatus for paint hook cleaning.
The present inventions address the foregoing objects in a direct manner by eliminating contact between titanium dioxide bearing paint and the surface of the product support hook. The apparatus of the present inventions comprises an electrostatic painting enclosure with a conveyor, product support hooks for the conveyor, an electrically conductive primer paint with a combustible base, for pre-coating the product support hooks, and preferably, a burn-off oven. While the other elements of the present invention are well known to those knowledgeable in the painting arts, the application of an electrically conductive coat of paint with a combustible base is unique. This primer paint may have up to 80% by volume of finely divided carbon or other suitable conductive material in a combustible, hardening base. Virtually any polymeric, epoxy or other conventional paint base, well known in the painting arts may be used to make this paint combustible as well as conductive. The coating may be applied in liquid or powder form by any of the means known to those skilled in the painting arts.
During electrostatic painting, a non-conductive coating of paint will build-up on the product support hooks, interfering with the product-to-hook ground connection in the next cycle of use. The paint build-up and combustible base coat are turned into ash in a burn-off oven and fall easily from the product support hooks. The primer coating prevents direct contact of the paint build-up with the hook surface, so that the titanium dioxide residue falls away with the ash. In this manner, good electrical conductivity is assured for reuse of the paint hooks. If the same product is to be painted throughout an extended production run, so that the product support hooks need not be changed for reasons other than cleaning, the burn-off oven may be incorporated into the conveyor path so as to provide a continuous process line. Even if a burn-off oven is not used, the conductive, combustible base coat isolates the titanium dioxides of pigmented paints from the paint hook surfaces, so as to facilitate cleaning by any other means.